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HomeMy WebLinkAbout009 Appendix H - Stormwater Management ReportEngineering Report Stormwater Design Stockyard Property October 2024 Revised June 2025 Submitted to: City of Bozeman Engineering Department 20 East Olive Bozeman, MT 59715 Prepared by: Stahly Engineering and Associates 851 Bridger Drive, Suite 1 Bozeman, MT 59715 (406) 522-9526 06/13/2025 Stormwater Plan 1 6/13/2025 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities Conveyance Capacity Facilities Maintenance Figures and Tables Figure 1. Vicinity Map Table 1. Storm Runoff Calculations Table 2. Retention Basin Capacities Table 3. Curb and Gutter Conveyance Calculations Table 4 Inlet Bypass and Curb Chase Capacity Calculations Table 5. Pipe Conveyance Calculations Attachments/References Grading & Drainage Plan C5.1 Stormwater Details Inlet Capacity Calculations Geotechnical Report Groundwater Monitoring Story Mill Park Infrastructure Storm Water Plan Stormwater Plan 2 6/13/2025 11.1 Engineering Report – General Information The Stockyard Property is located west of the intersection of East Griffin Drive and Story Mill Road. The Stockyard Property is a total of 15.28 acres comprised of 6 tracts with REMU and B-2M zoning. Water and sewer service for the property will be provided by extensions from existing City of Bozeman water and sewer mains adjacent to the site. The developer’s contact information is: Wake Up, Inc. Christine & Kevin Huyser P.O. Box 23 Harrison, MT 59735 The Stockyard Property is located as shown on the Vicinity Map in Figure 1. Figure 1: Vicinity Map, Bozeman, Montana Stormwater Plan 3 6/13/2025 Existing Site Conditions The Stockyard project is located in the northwest and southwest quadrants at the intersection of East Griffin Dr and Story Mill Rd. The property was previously occupied by an auction house, restaurant, several single-family residences and associated outbuildings. The site is gently sloping to the west and south. A geotechnical investigation was conducted, and the report is provided as an attachment to the infrastructure submittal. In summary the geotechnical investigation found a variable depth of fine-grained soils overlying gravel sediments. The depth to gravel varies between 3 and 13 feet. Groundwater monitoring was conducted throughout the 2022 season and found seasonal high groundwater depths ranged from 2’ to greater than 13’ below existing grade. Groundwater is flowing to the west and is assumed to discharge at the East Gallatin River which is the western boundary of the southern part of this project. The range of groundwater depth encountered was largely due to the rolling topography of the site with some wells located at lower or higher relative locations. After site grading, high groundwater will more consistently be about 10’ below the surface. Storm Drainage Plan Description The storm drainage plan for the Stockyard Property consists of separate systems for public and private property. Storm runoff from individual lots will be mitigated on-site as determined during each lot’s site design. The relatively deep depth to groundwater and conductive soils supports sub-surface infiltration of storm runoff, reducing impacts to surface waters and maintaining groundwater hydrology. Stormwater on this site falls into one of four categories. The first category includes stormwater from the northwestern portion of the site which will drain to street-side bio-swales along Stockyard Way for retention & infiltration. The second of these categories includes runoff from central and eastern portion of the site which will be collected and conveyed via pipe network to the common retention pond located in Tract 5. The third category includes runoff from Tract 5 which will flow overland to the retention pond. The fourth and final category includes storm water that falls on Tract 6. Stormwater from Tract 6 will be mitigated on-site upon site development. The stormwater retention system is sized to contain the storm runoff from the 10- year 2-hour storm, in accordance with City of Bozeman design standards. Storm drainage pipes are sized to convey the peak flow from a 25-year storm in accordance with City of Bozeman design standards. Private on-site stormwater Stormwater Plan 4 6/13/2025 systems will also be required to meet the City’s design standard for on-site stormwater. The drainage improvements are depicted on the Grading and Drainage Plan, attached to this report. The plan shows proposed grading, drainage areas, and stormwater facilities. Estimation of Runoff Runoff estimates were obtained for each sub-basin using City of Bozeman standards. For the stormwater retention calculations, the design storm is a 10- year, 2-hour storm. The Bozeman IDF (Intensity, Duration, and Frequency) curves show a rainfall intensity of 0.41 in/hour for the 10-year, 2-hour storm, totaling a 0.82-inch storm event. This exceeds the 0.5-inch event commonly used for water quality consideration. Additionally, the peak flow from the 25-year storm event is determined for purpose of purpose of sizing conveyance facilities. With a time of concentration of 10.9 minutes, the 25-year storm has an intensity of 2.32 in/hour, but due to its short duration, only represents a 0.32-inch event. For Table 1 shows the 25-year storm runoff calculations for the 5 reaches or sub- basins of the storm drainage network. Table 1 – Storm Runoff Calculations Conveyance Sub Areas 1 2 3 4 5 Sub Area Description Tract 5 West Half of E Griffin Dr & Tracts 2, 3, & 4 Story Mill Rd & East Half of E Griffin Dr Stockyard Way & Tract 1 W. End of E. Griffin Dr. Drainage Area (acres)2.58 2.63 1.81 1.72 0.19 Drainage Area (sf)112,596 114,348 78,887 74,878 8,074 Slope (%)1.5 1.5 1.5 1.5 1.5 Runoff Coefficients 0.65 0.65 0.65 0.65 0.9 Frequency Adjustment Factor, Cf 1.1 1.1 1.1 1.1 1.1 Basin Length (ft)300 300 300 300 150 Time of Concentration (min)10.9 10.9 10.9 10.9 2.2 Peak Flow Calculations Design Storm 25yr 25yr 25yr 25yr 25yr Time of Concentration (min)10.9 10.9 10.9 10.9 2.2 Intensity at Tc (Figure I-2 pg. 29) (in/hr)2.32 2.32 2.32 2.32 6.47 Peak Runoff Rate at Tc (Q = CIA) (cfs)3.91 3.97 2.74 2.60 1.08 Stormwater Plan 5 6/13/2025 Retention Facilities Stormwater from Subareas 1, 2, and 3 is mitigated by a common retention pond. This system is located on Tract 5 which is at the downgradient end of the property. Stormwater from Subarea 4 is mitigated by a streetside bio-swales along Stockyard Way. Due to grade constraints that prevent use of the proposed retention pond, stormwater from sub area 5 will be directed to existing bio-swales on the adjacent Story Mill Community Park property. The available capacity of these bio-swales was established in the 2017 Storm Water Plan for the Story Mill Park Infrastructure project which is attached to this report. To off-set the use of the existing facilities, the proposed retention pond on Tract 5 is sized to retain 1,100 cubic-feet more than the required 10-yr 2-hr storm. Table 2 describes the volume of storage required (using 10-year, 2-hr storm) for retention and storage capacity of the proposed retention systems. The available storage capacity of the existing bio-swales exceeds the volume required. Groundwater monitoring was conducted during the 2022 season and the bottom of the stormwater retention system is 3 feet above the seasonal high groundwater elevation. Additional construction details of the proposed retention system are provided in the Civil plan set. Table 2 – Retention System Capacities Drainage Basin Subareas 1, 2, & 3 4 5 Area (square feet)305,824 74,878 8,074 Area (acres)7.02 1.72 0.19 C Value 0.65 0.65 0.90 Rainfall (in) (10yr-2hr)0.82 0.82 0.82 Volume Req'd (cf)13,584 3326 497 Previously Allocated Volume (ft³)514 Total Volume Req'd (ft³)1011 Basin ID 1 2 Mitigation Type Retention Pond Bio-Swale(s)Ex. Bio-Swale(s) Gravel Bed Storage 1 2 Average Surface Area 0 4524 Depth 0 2 Total Cube Volume 0 9048 Gravel porosity 0.30 0.30 Gravel Storage (cf)0 2714 560 Above Ground Ponding 1 2 5 Top Area 12615 4524 Bottom Area 6955 4524 Depth 1.5 0.25 Total Ponding Storage (ft³)14677.5 1131 794 System Storage Capacity (ft³)14,678 3,845 1,354 Remaining Storage Capacity (ft³)840 Stormwater Plan 6 6/13/2025 Conveyance Capacity Conveyance capacity is determined using the peak flow from the 25-year storm shown in Table 1. Curb and gutter collects storm runoff and conveys runoff into the underground storm drainage pipe network. The capacity of a typical City standard curb and gutter at the minimum slope of 0.5% is shown in Table 3 below. A typical curb and gutter has a conveyance capacity of 5.04 cfs at a flow depth of 0.35-feet, which exceeds the 25-year peak flow from any single sub- basin as shown in Table 1. Table 3 – Curb and Gutter Conveyance Calculations Curb and Gutter Capacity Calcs Right-side Slope X:1 0.06 Left-side Slope X:1 33.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 0.35 Flow Area (ft^2) 2.025 Wetted Perimeter (ft) 11.906 Width 0.044 Hydraulic Radius (ft) 0.170 Manning's Roughness 0.013 Slope (ft/ft) 0.005 Average Velocity (ft/sec) 2.49 Flow (ft^3/sec) 5.04 Inlet capacities were calculated to determine the by-passed flow at each inlet. For this analysis, sub area 3 was divided into 8 smaller basins reflecting the 8 curb inlets along Story Mill Rd. and E. Griffin Dr.. The calculations below in Table 4 include only 4 of the basins since there is equal runoff from both the north and south halves. The peak flow for each area was added to any by-passed flow from the upstream basin. The total by-passed flow through the first 3 areas is 0.20 cfs as shown in Table 4. This was added to the peak flow for the final area which drains to the existing bio-swales via two curb chases. Capacity calculations for these curb chases are also included in Table 4 and show a capacity of 1.22 cfs at a flow depth of 4” for each chase. Further detail supporting these capacity calculations can be found attached to this report. The impact of the 25-year event on the existing bioswales was also considered. Since this is a lower volume event than the 10-yr 2-hr storm, the capacity of the bioswales is adequate for the 25-year event as well. Stormwater Plan 7 6/13/2025 Table 4 – Inlet Bypass and Curb Chase Capacity Calculations The storm drain piping collects runoff from sub areas 2 and 3 as described in Table 1. The 25-yr storm was used for conveyance requirements. Table 5 below shows the combined storm runoff for each of the four reaches. Table 5 also shows the minimum pipe sizes and slopes of each reach required to carry the storm runoff at that point in the system. Subarea Description SM West E Griffin South A E Griffin South B E Griffin South C Drainage Area (acres)0.46 0.33 0.13 0.08 Drainage Area (sf)20,002 14,286 5,682 3,613 Slope (%)1.5 1.5 1.5 1.5 Runoff Coefficients 0.65 0.65 0.65 0.65 Frequency Adjustment Factor, Cf 1.1 1.1 1.1 1.1 Basin Length (ft)300 300 300 300 Time of Concentration (min)10.9 10.9 10.9 10.9 Peak Flow Calculations Design Storm 25yr 25yr 25yr 25yr Time of Concentration (min)10.9 10.9 10.9 10.9 Intensity at Tc (Figure I-2 pg. 29) (in/hr)2.32 2.32 2.32 2.32 Peak Runoff Rate at Tc (Q = CIA) (cfs)0.69 0.50 0.20 0.13 By-pass flow from upstream basin (cfs)0.00 0.13 0.10 0.02 Total Flow (cfs)0.69 0.63 0.30 0.15 Inlet Capacity Calculations Inlet Type Combination Combination Combination 1.25' x 6" Curb Chase Channel Description Location On-grade On-grade On-grade 1.25 Channel Width (ft) Length (ft)3 3 3 0.050 slope (ft/ft) Throat ht (in) 5.00 5.00 5.00 0.013 coefficient of roughness (n) Open Area (sft)2.10 2.10 2.10 1 Depth in Channel (in) Grate Width (ft)1.5 1.5 1.5 1.42 wetted perimeter (ft) Grate Length (ft)3 3 3 0.10 area of flow (sf) Local Depression (in)0.00 0.00 0.00 0.07 hydraulic radius (ft) Slope, Sw (ft/ft)0.04 0.04 0.04 4.49 Avg. Velocity Slope, Sw (ft/ft)0.03 0.03 0.03 0.47 Channel Flowrate (cfs) Gutter Depression (in)0.18 0.18 0.18 Width (ft)1.50 1.50 1.50 Slope(%)0.50 2.40 2.40 n-value 0.013 0.013 0.013 Captured Flow (cfs)0.56 0.53 0.28 By-pass (cfs)0.13 0.10 0.02 *Inlet by-pass calculations were performed using Hydraflow Express. Output reports of these calculations are attached to this report. Stormwater Plan 8 6/13/2025 Table 5 – Pipe Conveyance Calculations Facilities Maintenance The proposed stormwater facilities including the retention pond on tract 5 and the bio-swales along Stockyard Way will be monitored and maintained by the Stockyard Property Owners’ Association. Monitoring and maintenance requirements will be provided in the owners’ association documents and are excerpted here for review. Storm Water Facilities Maintenance Schedule 1. Site Housekeeping. (Continuously as needed) The main cause of storm water facility damage is poor site housekeeping. Sediment tracked onto pavement can be washed into storm water bio- retention basins and conveyance piping and damage these facilities. Trash can clog pipes and inlet structures causing property damage. • Keep sidewalk and pavement areas clean • Pick up trash • Restore damaged landscaping in order to prevent sediment runoff 2. System Monitoring. (Quarterly, except in winter) The storm water facilities shall be inspected quarterly to quickly identify small issues before expensive damage can occur. In addition to regular monitoring, the best time to inspect the performance of storm water facilities is during runoff events. • Observe system during runoff. Look for ponding outside of retention areas. This can indicate a clogged inlet or pipe. • Inspect Bio-retention and Retention Basins • Inspect Inlets, Manholes and Pipes Pipe Location Retention System Inlet Pipe E. Griffin W. End E. Griffin East End 18" Stub to Sub Area 2 Pipe Material RCAP RCP PVC PVC Pipe Size (in)24 18.00 15.00 18.00 Manning's "n" (Concrete)0.013 0.013 0.013 0.013 Area (ft2)3.14 1.77 1.23 1.77 Wetted Perimeter (ft)6.28 4.71 3.93 4.71 Hydraulic Radius (ft)0.50 0.38 0.31 0.38 Slope (ft/ft)0.0033 0.0075 0.0050 0.0033 Full Flow Capacity (cfs)13.03 9.12 4.58 6.05 Velocity (ft/sec)4.15 5.16 3.73 3.42 Contributing Sub Area's 2,3 2,3 3 2 Stormwater Flow (cfs)6.70 6.70 2.74 3.97 % Capacity 51%73%60%66% Stormwater Plan 9 6/13/2025 3. Bio-retention and Retention Basin Maintenance. (Quarterly) The bio-retention and retention basins are designed to provide long-term sustainable treatment of storm water. However, with poor housekeeping sediment can clog these facilities and reduce infiltration capacity. Proper and timely cleaning of incidental sediment in these basins can prevent these facilities from being damaged. The bio-retention and retention basins are intended to be vegetated, since the vegetation provides microbiological communities that can treat common pollutants in storm water. Unhealthy vegetation can diminish the performance of these basins. The landscaping of the bio-retention basins shall follow typical landscape maintenance guidelines. The vegetation on the bottom of the retention basin should not be regularly mowed, since mowing can cause plant debris to build up in the basin. • Remove sediment, trash, and debris • Inspect for healthy vegetation • Inspect for uniform ponding and water disappears in 3 days 4. Bio-retention and Retention Basin Maintenance. (Annually) The vegetation should be maintained annually to reduce plant debris build-up. The retention basin vegetation should be cut to a height of 6” in the fall with the clippings removed from the basin. • Remove dead plant materials from bio-retention basins. • Cut retention basin vegetation and remove clippings. • Clean inlet piping or chases, remove sediment if present. 5. Bio-retention and Retention Basin Maintenance. (Long-term) If regular housekeeping and maintenance is not performed adequately, sediment and debris can accumulate in the basin and reduce the required storage capacity. If this occurs the basins should be excavated back down to original plan grade. • Hire a contractor to return basin(s) condition to initial design found on City engineering plans. • Dredge basin if sediment build-up is greater than 6” EAST GRIFFIN DRIVE STORY M IL L ROAD HILLSIDE LANE STORY MILLCOMMUNITY PARKSTORY MILLCOMMUNITY PARKHILLSIDE LANE TRACT 21.08 ACRESTRACT 11.30 ACRESTRACT 41.06 ACRESTRACT 53.86 ACRESTRACT 66.11 ACRESROW1.28ACRESTRACT 30.58 ACRESSTORY MILLCOMMUNITY PARKSUB AREA 41.72 ACRESSUB AREA 22.63 ACRESSUB AREA 12.58 ACRESSUB AREA 31.81 ACRESEXISTING OFF-SITE SUBAREA TO EX. BIOSWALES(0.28 ACRES)SUB AREA 5(ADDTIONAL 0.19 ACRESTO EX. BIOSWALES)1 OF 1GRADING &DRAINAGE PLAN1 1/16/2023 ROAD & UTILITY PLAN CDP ZWL 2 10/30/2024 COB INFRASTRUCTURE WLN CDP 3 2/14/2025 COB INF. RC1 WLN CDP 4 6/13/2025 COB INF. RC2 WLN CDP PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESSTOCKYARD SITEROAD & UTILITY PROJECT WAKE UP, INCBOZEMAN, MONTANA NOTESNOTES NOTES STORM PIPE MATERIAL SPECIFICATIONS NOTES CURB CHASE & SCUPPER DETAIL NOTES SIDE VIEWCHASE SECTION C5.1 STORMWATER DETAILS11/16/2023ROAD & UTILITY PLANCDPZWL210/30/2024COB INFRASTRUCTUREWLNCDP32/14/2025COB INF. RC1WLNCDP46/13/2025COB INF. RC2WLNCDPPROFESSIONALENGINEERS &SURVEYORS STAHLY ENGINEERING & ASSOCIATES STOCKYARD SITEROAD & UTILITY PROJECTWAKE UP, INCBOZEMAN, MONTANA